1. Field of the Invention
The present invention relates to a system for feedback-controlling the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine in which a plurality of cylinders are classified into two groups, to which exhaust passages are assigned individually.
2. Description of the Related Art
In an internal combustion engine to which the feedback control system of this type is applied, cylinders of two groups are separately connected to first and second exhaust passages which are independent of each other. The first and second exhaust passages open to the atmosphere through a common exhaust passage. An exhaust gas purifier or catalytic converter, which contains a three way catalyst, is disposed in the middle of the common exhaust passage.
The feedback control system, which is intended to control the air-fuel ratio of an air-fuel mixture to be supplied to the engine, comprises a first oxygen sensor for detecting the oxygen concentrations of exhaust gas discharged into the first and second exhaust passages, and a second oxygen sensor located on the lower-course side of the catalytic converter, in the common exhaust passage, and adapted to detect the oxygen concentration of the exhaust gas purified by the converter. Thus, according to the feedback control system provided with the first and second oxygen sensors, the amount of fuel to be supplied to each cylinder of the internal combustion engine, that is, the air-fuel ratio of the air-fuel mixture, is controlled in accordance with the oxygen concentrations of the exhaust gas detected by means of the first and second oxygen sensors. In this manner, the exhaust gas purifying efficiency of the catalytic converter can be improved.
In the feedback control system described above, the first oxygen sensor is generally disposed in the common exhaust passage on the upper-course side of the catalytic converter for efficaciously detecting the oxygen concentration of the exhaust gas flowing through the first and second exhaust passages by means of the first oxygen sensor.
In order to improve the exhaust gas purifying efficiency of the catalytic converter by means of the first and second oxygen sensors, it is necessary first to prevent a response delay of the first oxygen sensor and reduce the so-called limit cycle of feedback control. The response delay of the first oxygen sensor is caused, since the exhaust gas discharged from the engine takes much time to actually reach the first oxygen sensor after passing through the first and second exhaust passages and the common exhaust passage. Therefore, the response delay of the first oxygen sensor can be eliminated by disposing the first oxygen sensor in the exhaust passage so as to be situated as close to the engine as possible. However, since the exhaust passage of the engine includes the first and second exhaust passages separate from each other, as mentioned before, the first oxygen sensor must be disposed in only one of these two exhaust passages so as to be situated close to the engine. In this case, the first oxygen sensor can detect only the oxygen concentration of the exhaust gas discharged from the engine into the one exhaust passage. Accordingly, the amount of fuel to ber supplied to that group of cylinders connected to the other exhaust passage, that is, the air-fuel ratio of the air-fuel mixture for the cylinder group, cannot be feedback-controlled with high accuracy. Thus, the catalytic converter cannot enjoy a high purifying efficiency.
The first oxygen sensor may be disposed in each of the first and second exhaust passages so as to be situated close to the engine. This arrangement, however, requires an additional first oxygen sensor, thus entailing complicated feedback control, as well as increased costs of the feedback control system.
These problems associated with the arrangement of the first oxygen sensor are particularly noticeable when the feedback control system is applied to a V-type multicylinder internal combustion engine in which the first and second exhaust passages must inevitably be long.